feat(landing): add feature showcase and how it works sections

[Bhavy12-cell]

Closes #1773

Enhanced the landing page with new sections to better communicate
DevTrack's value to new visitors.

Changes made:

• Added FeatureShowcase.tsx with 6 feature cards (Commit Streak, PR Analytics, Goal Tracker, Leaderboard, AI Mentor, Heatmap)
• Added "How It Works" section with 3 steps (Connect GitHub → View Metrics → Set Goals)
• Scroll reveal animations on all new sections
• Fully responsive grid layout using auto-fit minmax
• Consistent with existing design system (fonts, colors, style).

[vercel]

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[github-actions]

GSSoC Label Checklist 🏷️

@Priyanshu-byte-coder — please apply the appropriate labels before merging:

Difficulty (pick one):

• level:beginner — 20 pts
• level:intermediate — 35 pts
• level:advanced — 55 pts
• level:critical — 80 pts

Quality (optional):

• quality:clean — ×1.2 multiplier
• quality:exceptional — ×1.5 multiplier

Validation (required to score):

• gssoc:approved — counts for points
• gssoc:invalid / gssoc:spam / gssoc:ai-slop — does not score

Type labels (type:*) are auto-detected from files and title. Review and adjust if needed.
Points formula: (difficulty × quality_multiplier) + type_bonus

[github-actions]

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A maintainer will review it within 48 hours. While you wait:

• Make sure CI is passing (type-check + lint)
• Double-check the PR description is filled out and the issue is linked
• Feel free to ask questions in Discussions if you need help

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[Priyanshu-byte-coder]

CI passed but this PR has merge conflicts with main. Please rebase:

git fetch origin
git rebase origin/main
git push --force-with-lease

[Priyanshu-byte-coder]

This PR has merge conflicts with main — please rebase:

git fetch origin
git rebase origin/main
# Resolve any conflicts, then:
git push --force-with-lease